1. Field of the Invention
The present invention relates to an optical information recording and reproducing apparatus, and more particularly, to an optical information recording and reproducing apparatus for recording information with respect to a recording medium on which information is recorded using holography, and for reproducing information from the recording medium having information recorded thereon.
2. Description of the Related Art
FIG. 39 is a block diagram showing an optical system of a conventional coaxial type (collinear type) holographic memory.
First, a description is given on a case where information recording is performed with respect to a hologram disk 216 serving as a recording medium.
A light beam outputted from a green laser 201 of a light source is collimated by a collimator 202, and irradiates a spatial light modulator SLM 204 via a mirror 203.
In FIG. 39, a deformable mirror device (DMD) is used for the SLM 204.
Light reflected by a pixel representing the information “1” on the SLM 204 is reflected to the hologram disk 216, and light reflected by a pixel representing the information “0” is not reflected to the hologram disk 216.
Provided on the collinear SLM 204 are a portion for modulating an information light 206 and a portion for modulating a reference light 205 surrounding the information light 206 in an annular shape.
The reference light 205 and the information light 206 reflected by the pixel representing the information “1” on the SLM 204 are transmitted through a polarizing beam splitter PBS 207 as P-polarized light. After that, the reference light 205 and the information light 206 become incident upon the hologram disk 216 via a relay lens (1) 208, a mirror 209, a relay lens (2) 210, and a dichroic BS 211.
The reference light 205 and the information light 206 transmitted through a quarter wavelength plate QWP 212 and converted to circular polarized light (e.g., clockwise circular polarized light) are reflected by a mirror 213 to be incident upon an objective lens 214 with a focal distance f.
A pattern displayed on the SLM 204 by two relay lenses (1) and (2) forms an intermediate image at a distance f before the objective lens 214.
Thus, a so-called 4f optical system is configured, in which a pattern image (not shown) on the SLM, the objective lens 214, and the hologram disk 216 are placed with an interval f from each other.
The hologram disk 216 has a disk shape and is rotatably held on a spindle motor 215.
The reference light 205 and the information light 206 are condensed onto a recording medium (not shown) on the disk by the objective lens 214 and interfere with each other to form an interference fringe.
On a polymer material in a recording medium, an interference fringe pattern during this recording is recorded as a refractive index distribution, and a digital volume hologram is formed. Further, in the recording medium, a reflective film is provided.
A red laser 220 which has no effect on the photosensitivity of the recording medium is provided, in addition to a green laser 201 for performing recording and reproduction with respect to a hologram, whereby a displacement of the hologram disk 216 can be detected with high precision, with the above-mentioned reflective film being a reference surface.
As a result, even when axial deflection and radial runout occur in the hologram disk 216, a recording spot can be allowed to follow the recording medium surface dynamically using an optical servo technique, and an interference fringe pattern can be recorded with high precision.
The above-mentioned aspect will be described briefly below.
A linear polarized light beam outputted from the red laser 220 is transmitted through a beam splitter BS 221, and collimated by a lens 222. After that, the light beam is reflected by a mirror 223 and the dichroic BS 211, and travels to the hologram disk 216.
The light beam transmitted through the quarter wavelength plate QWP 212 and converted into circular polarized light (e.g., clockwise circular polarized light) is reflected by the mirror 213 to be incident upon the objective lens 214. After that, the light beam is condensed as a minute optical spot on a reflective surface of the hologram disk 216.
The reflected light beam becomes reverse circular polarized light (e.g., counterclockwise circular polarized light), and is incident upon the objective lens 214 again to be collimated. After that, the collimated light beam is reflected by the mirror 213, transmitted through the quarter wavelength plate QWP 212, and converted into a linear polarized light beam perpendicular to a forward path.
The light beam reflected by the dichroic BS 211 passes through the mirror 223 and the lens 222 in the same way as in the forward path, and is reflected by the beam splitter BS 221 to be guided to a photodetector 224.
The photodetector 224 has a plurality of light receiving surfaces (not shown) and can detect positional information on a reflective surface by a known method. Based on the detected positional information, the photodetector 224 can perform focusing and tracking of the objective lens 214.
Next, a case where recorded information is reproduced from the hologram disk 216 serving as a recording medium by using the above-mentioned optical system will be described.
The light beam outputted from the green laser 201 of a light source irradiates the spatial light modulator SLM 204 in the same way as in recording. During reproduction, only a portion modulating the reference light 205 on the SLM 204 represents the information “1”, and a portion modulating the information light 206 displays information “0”.
Thus, only a part of the reference light reflected by a pixel is reflected to the hologram 216, and the information light is not reflected to the hologram 216.
In the same way as in recording, the reference light 205 becomes circular polarized light (e.g., clockwise circular polarized light) to be condensed onto a recording medium (not shown) on a disk, and reproduces information light from the recorded inference fringe. The information light reflected by the reflective film in the recording medium becomes reverse circular polarized light (e.g., counterclockwise circular polarized light), and becomes incident upon the objective lens 214 again to be collimated. After that, the collimated light is reflected by the mirror 213, transmitted through the quarter wavelength plate QWP 212, and converted into a linear polarized light beam (S-polarized light) perpendicular to the forward path. At this time, an intermediate image of a display pattern of SLM reproduced at a distance f from the objective lens 214 is formed.
The light beam transmitted through the dichroic BS 211 is directed to the polarizing beam splitter PBS 207 via the relay lens (2) 210, the mirror 209, and the relay lens (1) 208.
The light beam reflected by the PBS forms an image again as an intermediate image of a display pattern of SLM at a position conjugate to the spatial light modulator SLM 204 by the relay lenses (2) and (1).
At this position, an aperture 217 is previously placed, and unnecessary reference light in the periphery of the information light is blocked. Through the lens 218, the re-formed intermediate image forms an SLM display pattern only of a portion of the information light on a complementary metal oxide semiconductor (CMOS) sensor 219. Consequently, unnecessary reference light is not incident upon the CMOS sensor 219, so a reproduced signal with a satisfactory S/N can be obtained.
Regarding the above-mentioned technology, the following document is referred to: “Measurement and Nano-control Technology Supporting Holographic Memory/HVD™” (Proceeding of 35th Meeting on Lightwave Sensing Technology (LST35-12) June, 2005, Shochi Tan and Hideyoshi Horigome).
As described above, in the collinear holographic memory, information light and reference light can perform recording and reproduction in the coaxial optical arrangement having no angles, using one objective lens. Therefore, compared with a two-axis two-light beam interference system, an optical system is simplified.
Further, owing to the medium configuration with a reflective film, an optical system can be arranged on one side of a disk.
However, it is necessary to align the shift, tilt, and rotation of the two-dimensional spatial light modulator SLM 204 and the CMOS sensor 219 with high precision, which makes it difficult to decrease cost.
Further, a relay lens system used for an optical system projects the pattern of the spatial light modulator SLM 204 onto the CMOS sensor 219 exactly, so an expensive lens with a reduced distortion aberration and a reduced field curvature is required.
On the other hand, as another holographic memory, there is a system of recording a hologram on a recording medium by allowing two light beams to interfere with each other, as shown in FIGS. 40A and 40B.
FIGS. 40A and 40B show systems at a time of recording and reproduction, respectively.
During recording, a light beam is divided into two light beams by a beam splitter (BS) 401. One of the light beams is allowed to be incident upon a recording medium 404 at a predetermined angle via a galvanometer mirror 402 and relay lenses 403 as reference light, and the other light beam divided by the BS 401 is modulated to a two-dimensional pattern in accordance with information by a spatial light modulator SLM 405 to be incident upon the recording medium 404 by an objective lens 406 as information light. Thus, the two-dimensional pattern modulated by the SLM 405 is formed on the recording medium 404 as an interference pattern, and recorded thereon. Then, the incident angle with respect to the recording medium 404 is changed by the galvanometer mirror 402, whereby multi-recording in accordance with the angle is performed.
During reproduction, a light beam traveling to the SLM 405 and the objective lens 406 is blocked, and the recording medium 404 is irradiated with only the above-mentioned reference light at a predetermined angle, whereby a diffracted light by a hologram formed in the previous recording is generated. The diffracted light is condensed by an objective lens 407 for detection to form an image on a CMOS sensor 408, and the information on the two-dimensional pattern formed by the SLM 405 during recording is reproduced.
However, according to the above-mentioned configuration, an optical system and the like are arranged on both sides of a recording medium, so there is a problem that an apparatus is enlarged. In order to solve the problem, an apparatus in which an optical system and the like are arranged on one side of the recording medium has been proposed so as to miniaturize the apparatus.
An example thereof is described in 2006 Optical Data Storage Topical Meeting Conference Proceedings MA1 “The InPhase Professional Archive Drive OMA: Design and Function” as shown in FIGS. 41A and 41B.
In the same way as the above, FIGS. 41A and 41B show apparatuses at a time of recording and reproduction, respectively.
First, the apparatus at a time of recording will be described.
Among light beams from a laser light source 411, a light beam transmitted through a polarizing beam splitter PBS 417 becomes reference light, and is reflected by a mirror 418, a mirror 419, and a galvanometer mirror 420 to be guided to scanning lenses 421. The scanning lenses 421 irradiate the guided reference light to a hologram recording medium 422.
Herein, an expander 412 adjusts a light beam diameter to a desired diameter. A pin-hole 413 is a spatial filter for adjusting a wavefront. A shutter 414 is prepared for the purpose of controlling the exposure time during recording. An apodizer 415 is a filter for making the intensity distribution of a light beam uniform in a plane. A half wavelength plate 416 can rotate variably, and changes the polarized direction of a light beam incident upon the PBS 417 during reproduction described later so as to prevent light transmitted through the PBS 417 from being generated.
On the other hand, among the light beams from the laser light source, a light beam reflected by the PBS 417 is reflected by a PBS 426 to be incident upon a reflection type liquid crystal device 427. The incident light beam has a polarized direction changed, and reflected while being two-dimensionally modulated in accordance with predetermined information to become information light. The generated information light is transmitted through the PBS 426, and is irradiated to a hologram recording medium 422 by an objective lens 431.
Herein, an expander 423 adjusts the diameter of a light beam to be information light. A phase mask 424 is a filter for eliminating the non-uniformity of an intensity distribution in the hologram recording medium on which the light beam is condensed by the objective lens. Relay lenses 425 superimpose a two-dimensional pattern image of the phase mask 424 on the reflection type liquid crystal device 427. The relay lens 428 forms a two-dimensional pattern image of the reflection type liquid crystal device 427 on which the two-dimensional pattern image of the phase mask 424 is superimposed on an incident side focal plane of the objective lens 431. A polytopic filter 429 is an aperture for restricting the mixing of reproducing light from outside of a desired hologram during hologram reproduction described later. A half wavelength plate 430 is capable of being switched, and is provided so as to change the polarized direction of a light beam from the hologram recording medium 422 during reproduction described later.
Next, the apparatus at a time of reproduction will be described.
During reproduction, the polarized direction of a light beam from the laser light source 411 is changed by the half wavelength plate 416, whereby the light beam is prevented from being reflected by the PBS 417. The light beam transmitted through the PBS 417 is reflected by the mirror 418, the mirror 419, and the galvanometer mirror 420, and guided to the scanning lenses 421 to be irradiated to the hologram recording medium 422 by the scanning lens. At this time, although a diffracted light by a hologram recorded on the hologram recording medium 422 is generated, the diffracted light is not used as information reproducing light.
The light beam transmitted through the hologram recording medium 422 is reflected by the galvanometer mirror 432, and becomes incident upon the hologram recording medium 422 as reference light. The incident angle of the reference light with respect to the hologram recording medium 422 is controlled by the galvanometer mirror 420 and the galvanometer mirror 432.
The reference light irradiated to the hologram recording medium 422 generates diffracted light by the hologram recorded on the hologram recording medium 422 to become information reproducing light. The information reproducing light is collected by the objective lens 431 and has its polarized direction changed by 90° by the half wavelength plate 430, and furthermore, is incident upon the PBS 426 under the condition that information reproducing light other than the light corresponding to the reference light incident angle controlled by the galvanometer mirror 420 and the galvanometer mirror 432 is removed by the polytopic filter 429. The light beam incident upon the PBS 426 has its polarized direction rotated by 90°, so it is reflected by the PBS 426 to be incident upon the CMOS sensor 433.
Thus, two-dimensional pattern information corresponding to a desired hologram recorded on the hologram recording medium 422 is reproduced.
As described above, an apparatus in which an optical system and the like are arranged on one side of a recording medium (only the galvanometer mirror 32 is placed on the opposite side) is realized.
However, the above-mentioned conventional example has the following problem.
That is, regarding the reflection type liquid crystal device 427 and the CMOS sensor 433, the relative shift, tilt, and rotation with respect to an optical axis are required to be aligned, which causes an increase in cost for adjusting an assembly. Further, the sizes of the reflection type liquid crystal device 427 and the CMOS sensor 433 are larger than those of a lens and the like that are the other constituent elements of the apparatus, which inhibits the miniaturization of the apparatus.
In view of the above-mentioned problems, it is an object of the present invention to alleviate the assembly adjustment precision regarding a spatial modulator and an image photodetector element and to further miniaturize an apparatus in a collinear system and a system to conduct recording by allowing two light beams to interfere with each other.